The invention relates to a brake force amplifier for the brake system of a motor vehicle, which is activated by a brake pedal by means of a travel-limiting flexible element and is provided with a control valve, which monitors both a connection from a pressure source to an augmenter cylinder that contains an augmenter piston, and a connection from the augmenter to a relief line. This type of brake force amplifier is known. These devices generally employ a control valve, which can be activated by the brake pedal by means of a so-called "travel-limiting flexible element" which includes an inlet and an outlet valve for the brake pressure medium.
In the released condition of these control valves, the inlet valve is generally closed, and the brake lines are relieved by means of the outlet valve, and in the braking condition the outlet valve is closed and the inlet valve opened. In order to make it possible to give the driver both a feel for the actuation of the brakes and an impression of the valve position attained, thereby informing him of the braking effectiveness, the travel-limiting flexible element is arranged between the brake pedal and the control valve. Thus a varying reaction force acts against the pedal pressure in dependence on the respective characteristic curve of the flexible element.
When the brakes are activated by the pedal the following activities occur between a pedal push rod piston and the control valve: The pedal push rod piston is pushed against the opposing force of the travel-limiting flexible element. In so doing, a sealing surface of a closing body comes into contact with an outlet seat, thus sealing the outlet valve. The travel path of the outlet valve closing body is designated as the outlet stroke.
The activating path of the pedal push rod piston, however, is greater than the outlet stroke, because the travel-limiting flexible element, which is generally relatively soft, must first be compressed to a certain degree before it sets the outlet valve in motion. In addition to the spring force, the frictional forces acting on the push rod piston, which is guided in a sealed manner, must also be overcome.
The arrival of the valve seat on the outlet valve sealing surface of the control valve does not immediately lead to an opening of the inlet valve as the push rod piston continues its motion, because the inlet valve, as a rule, is loaded by a biased spring having a certain closing force, and an additional force is exerted in the closing direction by the supply pressure in the chamber behind the inlet valve. To overcome these forces the travel-limiting flexible element is compressed by a certain additional amount, before the desired opening of the inlet valve is accomplished, thus introducing the associated brake pressure.
Thus, the travel-limiting flexible element must be capable of performing this entire travel stroke.
If, however, the auxiliary force fails, then the travel-limiting flexible element represents a wasted motion element, for when the auxiliary force is missing, the entire pedal travel path must be available for braking. Until now, an action to disengage the travel-limiting flexible element has required complicated measures that were susceptible to malfunctions.